Writing instruments containing an aqueous ink composition used to contain water, water soluble solvents and the like as a principal component, and therefore involved therein are the problems that when a long time passes while the pen tip part is left opened, a liquid component in the ink is vaporized to increase a viscosity of the ink and that the pen tip is dried to cause inferior writing.
A method for solving the above problems by adding glycols is usually known in a wide range, but it involves the defect that drying of the drawn lines is delayed.
An aqueous ink composition for solving the above problems by adding, for example, 1,4-cyclohexanedimethanol and lower alcohol is known (refer to, for example, a patent document 1).
However, it is the existing situation that in the technology shown in the patent document 1 described above, the ink is insufficient in a countermeasure for a stability at low temperature and brings about feathering and strike-through in the drawn lines and that a satisfactory effect is not obtained in drying of the drawn lines.
On the other hand, an aqueous ink containing at least a colorant, trimethylglycine (glycine betaine) and water is known as an ink which is excellent in a drought resistance of an ink discharge part such as a pen tip (refer to, for example, a patent document 2).
However, it involves the problem that in the technology shown in the patent document 2 described above, the ink is still insufficient in a drought resistance on the conditions of a high temperature and a high humidity and that a satisfactory effect is not obtained in drying of the drawn lines.
In particular, a wood free paper is used in increased cases for instructional books represented by textbooks in recent years. In general, the wood free paper has a property of being less liable to absorb water, and therefore the lines drawn on the paper are insufficiently dried to cause stain on the paper in certain cases. Also in the technologies shown in the patent documents 1 and 2 described above, involved therein are the problems that the lines drawn on the wood free paper are still insufficient in a drying property and that a drought resistance of the pen tip and a low temperature stability of the ink are unsatisfactory.